Method of electrophotography

ABSTRACT

IN A METHOD OF ELECTROGRAPHY COMPRISING THE STEPS OF APPLYING A FIRST FIELD OF ONE POLARITY ACROSS A PHOTOSENSITIVE ELEMENT INCLUDING A PHOTOCONDUCTIVE LAYER AND A HIGHLY INSULATED LAYER INTEGRALLY BONDED TO ONE SIDE OF THE PHOTOCONDUCTIVE LAYER AND APPLYING A SECOND FIELD OF OPPOSITE POLARITY CONCURRENTLY WITH OR BEFORE PROJECTION OF A LIGHT IMAGE WHEREBY TO FORM AN ELECTROSTATIC LATENT IMAGE CORRESPONDING TO THE LIGHT IMAGE ON THE SURFACE OF THE HIGHLY INSULATIVE LAYER, UNIFORM LIGHT IS PROJECTED ON THE PHOTOSENSITIVE ELEMENT ON THE SIDE OPPOSITE TO THAT OF THE HIGHLY INSULATIVE LAYER CONCURRENTLY WITH OR PRIOR OR SUBSEQUENT TO THE APPLICATION OF THE FIRST FIELD.

y 1972 KOICHI KINOSHITA 3,67

METHOD OF ELECTROPHOTOGRAPHY Filed Oct. 9, 1968 2 Sheets-Sheet 1 FIG. I

FIG. 2(0) F |G.3(b) FIG. 3(0) I l FIG. 3(c) Ti+? FIG- 3(a) f-1 y 1972 KOICHI KINOSHITA 5,

METHOD OF ELECTRQPHOTOGRAPHY Filed Oct. 9, 1968 z Sheets-Sheet 2 F16. 4( FIG. 4(b) United States Patent O US. Cl. 96-1 R 5 Claims ABSTRACT OF THE DISCLOSURE In a method of electrography comprising the steps of applying a first field of one polarity across a photosensitive element including a photoconductive layer and a highly insulated layer integrally bonded to one side of the photoconductive layer and applying a second field of opposite polarity concurrently with or before projection of a light image whereby to form an electrostatic latent image corresponding to the light image on the surface of the highly insulative layer, uniform light is projected on the photosensitive element on the side opposite to that of the highly insulative layer concurrently with or prior or subsequent to the application of the first field.

BACKGROUND OF THE INVENTION This invention relates to a method of electrophotography wherein deterioration of the image forming property caused by trapped charge established in the interior of a photoconductive layer of a photosensitive element can be prevented.

Since the development of a method of electrophotography employing a photosensitive element comprising a photoconductive layer such as a vapour deposited layer of selenium and an electrode (see US. Pat. No. 2,297,691) workers in the art have concentrated their interest on the thickness of the photosensitive element. Particularly in the so-called transfer printing type method of electrophotography in which the photosensitive element is used repeatedly, because the deterioration of the element due to hysteresis of the latent image and to repeated operations effected at a high speed is closely related to the thickness of the photosensitive layer, and because there is a limit on the thickness of the photosensitve layer imposed by manufacturing techniques, many problems are still unsolved. Also in a photosenstive element having an insulative protective film provided for the purpose of protecting the surface of the photosenstive layer and pre; venting deterioration of the electrostatic latent image formed thereon, these problems have not been solved.

In the method of electrophotography disclosed in US. Pat. No. 2,297,691, which is known as one of the most basic prior methods, vapour deposited layers of selenium are mainly used as the photosensitive layers. Where photosensitive elements utilizing selenium are uniformly charged in the dark and are then exposed to light, positive charge is selected. This polarity is selected by taking into consideration the ability of preserving the positive charge in the dark due to the fact that the majority carriers of selenium are holes. In selenium, since the mobility of electrons is low, holes of free charge pairs of holes and electrons formed by the excitation of heat or light can move freely but electrons cannot move appreciably. For this reason when the surface of the photosensitive element is charged positively in the dark, free holes created by heat are caused to migrate toward the backing electrode of the photosensitive element by the repulsive force created between free holes and the charge. On the other hand, while electrons can migrate toward the surface of the "ice photosensitive layer they have but little ability to combine with the charge. Accordingly, the electrostatic charge can be readily preserved in the dark. Thus, the holes created by thermal excitation are trapped in the impurity levels caused by defects in the body of selenium while they migrate through the photosensitive layer, thus forming a type of space charge. In the same manner, holes of the hole-electron pairs created by light excitation migrate toward the backing electrode and are trapped thereby. Ordinarily, as light excitation is applied to one surface only of the photosensitive layer, most of the incident light rays are absorbed on their way so that they do not reach the region close to the backing electrode thus resulting in a space charge of a plurality of majority carriers in this region.

It is clear that presence of such a space charge results in the change in the condition of subsequent charge and light illumination. Actually, this establishes a limit for repeated operations of said known basic method of electiophotography.

Further in the method utilizing a photosensitive element which is provided with a transparent highly insulative layer integrally bonded to the surface of the photosensitive layer for the purpose of protecting the same, it is required to increase the thickness of the photosensitive layer, thus increasing the tendency mentioned above. To solve this problem various means have been proposed. The basic concept of these prior proposals has been to extinguish the space charge formed by the charge trapped in the photosensitive layer by recombination efiected by injection of charge carriers from the electrode of the photosensitive layer of a polarity opposite to that of the space charge. For example, according to one method disclosed in US. Pat. No. 2,901,348, the above described problem was solved by adopting a particular construction of the photosensitive element whereas in the other method disclosed in US. Pat. No. 3,041,167 this problem was solved by the application of an electric field.

The result of experiment, however, showed that, in spite of these efforts, the space charge still remained significantly. Thus, more effective means of removing trapped space charge has been highly desired.

SUMMARY OF THE INVENTION It is therefore an object of this invention to eliminate residual charge which is objectionable if the electrophotographic elements are to be utilized repeatedly.

In accordance with this invention there is provided a new and improved method of electrophotography comprising the steps of applying a first field of one polarity across a photosensitive element including a photoconductive layer and a highly insulative layer integrally bonded to one side of the photoconductive layer, applying a second field including a component of the polarity opposite to said one polarity across the photosensitive element and projecting a light image on the photosensitive element from the side of the highly insulative layer concurrently with or after application of the second field whereby to form an electrostatic latent image corresponding to the light image on the surface of the highly insulative layer, characterized by a step of projecting uniform light on the side of the photosensitive element opposite to that projected with the light image concurrently with or prior or subsequent to the application of the first field.

BRIEF DESCRIPTION OF THE DRAWING The invention can be more fully understood from the following detailed description taken in connection with the accompanying drawing in which:

FIG. 1 is a perspective view of one example of a photosensitive element employed in the novel method of electrophotography;

\FIGS. 2a, 2b and 20 show successive steps of the novel method of electrophotography;

FIGS. 3a to 3 inclusive are diagrams to show the charge distribution according to the prior method of electrophotography;

FIGS. 4a to 4 inclusive are diagrams to show charge distribution according to the novel method of electro photography, and

FIG. 5 is a schematic representation of an apparatus utilized to continuously carry out the novel method.

DESCRIPTION OF 'ITE-IE PREFERRED EMBODIMENTS Example 1 FIG. 1 of the accompanying drawing illustrates one example of a photosensitive element utilized to carry out the novel method of electrophotography and comprising a backing electrode of a transparent electric conductor, such as Nesa glass (registered trademark) 1, a transparent layer 2 of a synthetic resin of polyester series, for example, having a thickness of 6 microns, for example, and integrally bonded to one side of the backing electrode 1, a photosensitive or photoconductive layer 3 of selenium having a thickness of 70 microns and integrally bonded to the transparent resin layer, and a transparent layer 4 of 10 microns thick preferably made of the same material as layer 2 and integrally bonded to the surface of photosensitive layer 3 to protect it.

By utilizing this photosensitive element, an electrostatic latent image was formed in the following manner.

As shown in FIG. 2a, a suitable source of direct current 20 was connected across backing electrode 1 and a corona discharge electrode 6, in the form of a fine metal wire, to apply a potential of minus 6,000 volts to the latter. Then the corona discharge electrode and the photosensitive element were moved relatively to scan the surface of the photosensitive element to charge negatively the surface of the highly insulative layer 4. Concurrently therewith, uniform light of a brightness of 20 luxes was projected upon the photosensitive element for 0.5 second from a source of light 5 through transparent backing electrode 1, thus completing the first step. When measured in the dark, the potential of the electrostatic charge on the surface of the highly insulative layer was approximately minus 550 volts. It was confirmed that substantially the same value can be obtained when the measurement is made under room light. As shown in FIG. 2b, in the second step, the polarity of the DC source 20 was reversed and the photosensitive element was uniformly charged positively with corona discharge in the same manner as in FIG. 2a until the potential of the surface of the highly insulative layer attained plus 500 volts.

Then as shown in FIG. 20, in the third step, a light image of an object 21 having a brightness of 10 luxes at its bright portions was projected upon the photosensitive element through the highly insulative surface layer 4 for 0.5 second to form an electrostatic latent image on the surface of highly insulative layer 4 having potentials of plus 500 volts at portions corresponding to dark portions of the light image and of minus 20 volts at portions corresponding to bright portions of the light image. When developed by a suitable developer in the form of charged fine powder commonly used in the art of electrophotography, this latent image gave an intense visible image. The latent or developed image can be transfer printed on any suitable recording medium by the well known method. Subsequent to the transfer printing, the photosensitive element was cleaned by a suitable brush and immediately thereafter the above described cycle of operation was repeated without any hysteresis of the previously formed latent image. The potential difference between portions corresponding to bright and dark portions of the light image was the same as that obtained in the first cycle, thus showing no degradation of the quality of the latent image.

For comparison, instead of projecting uniform light through the transparent backing electrode, when uniform light was projected upon the photosensitive element through the transparent highly insulative layer 4 in the first step, the potential at portions of the latent image corresponding to the dark portions of the light image, formed by the first image forming cycle, Was plus 500 volts whereas that at portions corresponding to bright portions of the light image was plus 270 volts. After several tens of cycles of repeated operations the potential at portions corresponding to dark portions of the light image remained at plus 500 volts but that corresponding to bright portions had increased to plus 480 volts. This decrease in the potential difference between bright and dark portions means substantial loss of the image forming ability. Repetition of the image forming cycles was performed at a regular rate of one cycle every 20 seconds.

The theory of the significant effect caused by the illumination of uniform light from the rear surface of the photosensitive element described in the above example is believed to be explained as follows.

FIGS. 3a to 3 show diagrams to explain the manner of charge shift or migration in the method of forming a latent image in which instead of projecting uniform light through the backing electrode uniform light was projected upon the surface of the photosensitive element.

FIG. 3a shows a state wherein negative charge was uniformly applied on the surface of the photosensitive element in the dark. Under this condition, as there is no light excitation, the photosensitive layer acts as an insulator. FIG. 3b shows a state wherein uniform light was projected upon the uniformly charged photosensitive element through the highly insulative surface layer thereof. Considering a photosensitive material consisting of selenium utilized in the above example, as selenium is a P-type semiconductor, the mobility of holes in selenium is extremely greater than that of electrons so that electrons can not migrate over a large distance.

On the other hand, a selenium layer can best absorb light rays of wavelengths of about 500 millimicrons to 600 millimicrons and since light rays in this absorption range alone contribute to the photoconductive phenomenon of the selenium layer light rays incident upon one surface of the selenium layer are abundantly absorbed at the surface of their impingement and gradually decay away as they penetrate into the interior of the selenium layer. For this reason, light excitation is concentrated at the surface portion of the selenium layer and the bottom portion thereof operates as an insulator, as best shown in FIG. 3b.

Electron-hole pairs created in portions of the selenium excited by light migrate to respective sides of opposite polarity in accordance with the influence of the electric field created by the charge previously applied upon the photosensitive layer. However, due to said large difference in the mobility of electrons and holes mentioned above the distance through which electrons migrate toward the backing electrode due to its attraction is small. While holes have large modibility, the actual distance of their travel is limited because they are blocked by the highly insulative layer on the surface of the photosensitive element. 'Ihus holes are trapped by trap levels near the current blocking layer to establish a charge distribution as shown in FIG. 3b.

Where the polarity of the electrostatic charge on the surface of the photosensitive element is reversed in the dark, if the trapping level of the charge is at a suitable depth the charge that has been trapped by the process shown in FIG. 3b has little chance of being released unless excited by light, so that a charge distribution as shown in FIG. 30 will result.

The variation of the charge distribution occuring when a local illumination is provided through the highly insulative layer of the photosensitive element is just the opposite as that shown in FIG. 3b. More particularly, holes of electron-hole pairs created near the surface of the selenium layer by light illumination migrate over large distances toward the backing electrode and are trapped in the region close thereto while electrons remain near the surface of the selenium layer.

FIG. 3d shows such a local change in the charge distribution. As can be readily understood from FIG. 3d, under the state wherein the surface of the highly insulative layer of the photosensitive element is charged positively migration of charge in response to light illumination is large and hence in the step shown in FIG. 3d the surface potential of the photosensitive element changes greatly in response to projection of the light image, thus forming a so-called electrostatic latent image.

However, when the same steps as those shown in FIGS. 3a through 3d are repeated different phenomena occur. FIG. 32 shows a charge distribution of step identical to that shown in FIG. 3a but performed in the next subsequent cycle and shows the residual influence of the light image that was projected during previous steps. This is because a not readily releasable trapped charge is formed owing to the difference between the mobilities of majority carriers and minority carriers.

It will be clear that such hysteresis of the image formed also exists in the step shown in FIG. 3 which corresponds to FIG. 3b. Consequently, the hysteresis of the image formed in the first cycle appears as superposed upon the image formed in the second cycle.

When such trapped charges having no chance of release are permitted to accumulate as a result of repeated image forming operations an internal field of a definite value will be created in the photosensitive layer due to the positive charge accumulating near the electrode of the photosensitive element whereby response on light illumination of the photosensitive layer is impaired thus losing the image forming ability thereof. Such phenomenon can be eliminated by the projecting uniform light to the photosensitive element from its backing electrode side according to the teaching of this invention.

FIGS. 4a through 41 show charge distribution of various steps identical to those shown in FIGS. 3a through 3 wherein uniform light is irradiated through the backing electrode at the time of applying the uniform charge in accordance with this invention.

FIG. 4a is identical to FIG. 3a and FIG. 4b shows the charge distribution of the case wherein uniform light is projected upon the photosensitive element through the backing electrode as shown by arrows. As described hereinabove, where the positive and negative free charges pairs are formed near the backing electrode by light excitation positive charge having highere mobility migrate over a longer distance to be trapped near the upper current blocking layer. As shown in FIGS. 4c and 4d, at an instant when the image forming operation identical to that shown in FIG. 3d has been completed the positive charge comprising the majority carriers can migrate over a longer distance in response to light illumination, thus resulting in higher photosensitivity in various steps. Further, as shown in FIGS. 4e and 4 since trapped charge formed by the image forming operation in the previous cycle is perfectly released by the uniform light illumination through the backing electrode, charge distributions shown in FIGS. 4b and 4 are quite identical which shows that no hysteresis of the image occurs. For this reason even if cycles of image formation are repeated many times there is no fear of decreasing image forming ability due to the residual trapped charge as has been described in connection with FIG. 3. Thus, by the addition of a simple step of projecting uniform light through the backing electrode it is possible to obtain high photosensitivity and stable latent images free from the effect of hysteresis.

Although in the above described embodiment, a photosensitive element having highly insulative layers on both sides thereof has been illustrated it should be understood that this invention is also applicable with equal results to other types of photosensitive element, for example, those having no insulative layer between the photosensitive layer and the transparent backing electrode.

Example 2 A photosensitive layer was formed by vacuum depositing selenium to a thickness of 70 microns on one surface of a conductive glass plate at normal temperature and a synthetic resin of the polyester series having a thickness of 6 microns was integrally bonded to the surface of the photosensitive layer to obtain a photosensitive element.

The same process steps of electrophotography as those of Example 1 were applied to this photosensitive element. Thus, where uniform light was projected upon the element through its transparent backing electrode concurrently with the application of the first electric field, a latent image was obtained manifesting a potential of plus 450 volts at portions corresponding to dark portions of the light image. Even when this image forming operation was repeated several hundred times, the same result was obtained at each time, again free from any hysteresis of the image previously formed. As has been described in connection with Example 1, it was noted that where uniform light was irradiated through the upper highly insulative layer concurrently with the application of the first field strong hysteresis of the previously formed image was experienced thus substantially losing the image forming ability.

In the photosensitive element utilized in Example 2 as the backing electrode is in direct contact with the photosensitive layer, transfer of charge carriers occurs therebetween. Accordingly, judging from the manner of form,- ing trapped charge and the effect of charge carriers injected from the electrode it is reasonable to presume that there will be some difference between Examples 1 and 2, but unexpectedly, the results of experiments were substantially identical. Although the exact mechanism of this is not yet clear it may be considered as follows. Thus, even in the absence of the current blocking layer there exists a resistance between the selenium layer and the electrode which is sufliciently high to permit ready transfer of charge carriers in the dark thus assuring easy injection of charge carriers upon irradiation of light. If this assumption of the presence of a resistance which is varied by light illumination were correct, it would be clearly understood that the same explanation can be applied for both examples irrespective of the presence or absence of the current blocking layer.

It should be also be understood that this invention is not limited to any particular material or construction of the photosensitive elements illustrated in Examples 1 and 2. Thus, for example, the photosensitive layer may be made of any inorganic semiconductor such as CdSe, CdS, ZnO', ZnS, (ZnCd)S, Pbs, PbO etc., or any organic semiconductor such as anthracene, anthraquinone, polyvinylcarbazole and the like. In addition to vapour deposition technique referred to above, the photosensitive layer may be fabricated by any suitable methods including a method of forming a thin layer by using a suitable binder, and a method of chemically depositing the desired semiconductoron a substrate. Thus, any photoconductive material which can respond to light and has an ability of trapping electrical charge can be used in this invention.

FIG. 5 is a diagrammatic representation of a drum type apparatus employed to continuously carry out the novel method of electrophotography. In and around a photosensitive drum are disposed a source of uniform light 5, a negative corona discharge electrode 6, a positive corona discharge electrode 7, an optical system 8 for projecting a light image of an object 9, a magnetic brush for development 10, a transfer printing roller 12 and a cleaning brush 11, as shown in FIG. 5. The photosensitive drum may include a photosensitive element having a similar construction to that shown in FIG. 1 that is comprising a backing electrode conductive glass 1, highly insulative layers 2 and 4 and a photosensitive layer 3 sandwiched therebetween.

Although optical system 8 is shown as circumferentially displaced from positive corona discharge electrode 7, actually it is preferable to construct it such that the light image of object 9 is projected through corona discharge electrode 7 so that projection of the light image and application of the second field may be made simultaneously. Further, the polarity of corona discharge electrodes 6 and 7 can be selected suitably. It was also found that where an AC field is employed as the second field, substantially the same result was obtained. This is because half cycles of the AC field having the same polarity as the first field do not alter the distribution of charge and only the other half cycles of the opposite polarity contribute to the formation of the second field. Therefore, the second field may be a pure DC field or a pulsating or alternating field so long as it contains a field component of the opposite polarity as the first field.

In the foregoing examples it has been shown that the image forming property is excellent when the polarities of fields impressed during the first and second steps are selected such that majority carriers of the semiconductor can migrate over long distances during respective steps. However, the result of experiment showed that even when the polarities of the fields are reversed latent images can be formed to some extent. Especially, where the backing electrode is in direct contact with the photosensitive layer without the intervention of a current blocking layer considerable image forming ability can be provided. This can be understood when one considers the possibility of injection of charge carriers.

While in the illustrated examples the backing electrode has been shown as being integrally bonded to the photosensitive element such construction represents only one example of a plurality of means for applying electric field across the photosensitive element. For example, the backing electrode may be removably applied to the photosensitive element. Likewise a removable transparent electrode may be substituted for the corona electrodes to charge the surface of the highly insulative layer. Thin metal foil or transparent conductive sheet of paper or the like may be substituted for conductive glass.

Since the purpose of projecting uniform light on the photosensitive element from the side opposite to that of the element receiving the projection of light image is to create substantial migration of charge carriers the same result can be obtained whether the projection of uniform light is made concurrently with or subsequently to the application of the first field. Even when the uniform light is projected upon the element prior to the application of the first field similar result can be obtained owing to the phenomenon of the persistency of photoconductivity which is common to photoconductor bodies. Such method is also included in the scope of this invention.

I claim:

1. In a method of electrophotography comprising the steps of applying a first field of one polarity across a photosensitive element including a photoconductive layer and a transparent highly insulative layer integrally bonded to one side of said photoconductive layer, applying across photosensitive element a second field including at least a component of a polarity opposite to said one polarity, and projecting a light image onto said photoconductive layer through said insulative layer for cooperation with said second field to form an electrostatic latent image on the surface of said insulative layer, the improvement which comprises the steps of projecting uniform light upon said photoconductive layer from the side remote from said insulative layer for cooperation with said first field, said uniform light having an intensity sufficient to cause substantially complete separation of any majority and minority charge carriers in said photoconductive layer, said photoconductive layer having majority and minority charge carriers of diiferent mobility and a thickness such that said uniform light if projected through said transparent highly insulative layer does not cause substantial excitation of said photoconductive layer on the side opposite said highly insulative layer and selecting the polarity of said first field such that the side of said photosensitive element which receives said uniform light illumination has the same polarity applied to it as the majority carriers of the photoconductive layer.

2. The method of electrophotography according to claim 1 wherein said uniform light is projected concurrently with the application of said first field.

3. The method of electrophotography according to claim 1 wherein said uniform light is projected prior to the application of said first field.

4. The method of electrophotography according to claim 1 wherein a transparent conductive backing electrode is provided for said photosensitive element on the side op posite to said highly insulative layer and said uniform light is projected through said backing electrode.

5. The method of electrophotography according to claim 1, wherein said uniform light is projected subsequent to the application of said first field but before the application of said second field.

References Cited UNITED STATES PATENTS 2,833,648 5/1958 Walkup 96--1 3,457,070 7/1969 Watanabe et al. 96-1.4 3,041,167 6/1962 Blakney et al. 96l.4 3,268,331 8/1966 Harper 961 3,337,339 8/1967 Snelling 96-1 3,355,289 11/1967 Hall et al. 961.4 3,406,060 10/1968 Schlein et al 96-1 3,438,706 4/1969 Tanaka et al 355--11 OTHER REFERENCES Cassiers, Memory Effects in Electrophotography, J. of Photographic Science, vol. 10, pp. 57-64, (1962).

Shattuck et al., Presensitization of Organic Electrophotographic Layers by Exposure or Polarization and Exposure, IMB Technical Disclosure Bulletin, vol. 8, No. 5, pp. 717, 718 (October 1965).

C. E. VAN HORN, Primary Examiner 

